The candidate, Manisha Balwani MD, MS, is an Assistant Professor in the Department of Genetics and Genomic Sciences at the Mount Sinai School of Medicine and is board certified in Internal Medicine and Clinical Genetics. This grant proposal is designed to provide the candidate with a mentored training experience that will facilitate her development as an independent clinical researcher focused on the Porphyrias, the inborn errors of heme biosynthesis. The Mentors are experienced in the clinical, biochemical, and molecular studies of these diseases. The Mentors have a strong record in mentoring fellows and junior faculty in translational research, clinical trials and drug development. Thus, the mentoring program and laboratory and clinical research environment that will be available to the applicant will facilitate her development as an independent researcher. The proposed research will focus on the Erythropoietic Protoporphyria (EPP)-phenotype, a group of genetically heterogenous photo-induced, severe, cutaneous porphyrias. Three subtypes of the EPP-phenotype have been identified to date: 1) autosomal recessive EPP due to loss- of-function mutations in the ferrochelatase (FECH) gene, 2) X-linked Protoporphyria (XLP), a newly recognized subtype resulting from gain-of-function mutations of the X-linked erythroid-specific d- aminolevulinate synthase (ALAS2) gene, and 3) a subtype with elevated erythrocyte protoporphyrins, cutaneous photosensitivity, and normal FECH and ALAS2 alleles. The proposed studies will initially identify, characterize, and determine the frequency of the FECH and ALAS2 mutations causing the EPP-phenotype in over 100 unrelated patients already enrolled in the Porphyria Consortium. The natural history, clinical spectrum, quality of life, and erythrocyte protoporphyrin levels will be determined in patients with mutation- confirmed EPP and XLP. In XLP, the absence or presence and severity of clinical manifestations, and the levels of erythrocyte free- and zinc-protoporphyrins will be determined in female heterozygotes and correlated with the proportion of mutant ALAS2 alleles expressed in hematopoietic cells from individual heterozygotes due to skewing of random X-chromosomal inactivation. A novel FDA-approved pilot study will be conducted in patients with EPP and XLP to determine if Isoniazid, which binds to pyridoxal phosphate, a co-factor of ALAS2, can decrease the activity of ALAS2, thereby reducing the formation of erythrocyte protoporphyrin, which causes the XLP manifestations. The protected time afforded by this award would permit the applicant to become expert in the diagnosis and management of the porphyrias, facilitate the applicant's goal of establishing an independent research career focused on studies of porphyria pathogenesis and the development of novel treatments.